Heating objects using high frequency radiation is wide spread, and comprises the commonly used domestic microwave (MW) oven, as well as commercial ovens that use MW energy, mainly in combination with other means of heating, such as steam, hot air and infrared heating elements.
Among the many problems associated with known MW ovens is a lack of uniformity in heating, which often results in hot spots and cold spots that reflect the standing wave within the cavity. Many of the attempts to improve uniformity in such devices included increasing the number of modes within the cavity (e.g. by mode stirring and/or moving the load during heating).
In some cases, where multiple frequencies were used, the devices were configured to measure the efficiency of energy transfer into the cavity at different transmitted frequencies and then to transmit energy to the load only at frequencies having a relatively high efficiency, with the intent that this should increase the efficiency of energy transfer into the load.
Heating an object changes its dissipation characteristics at different frequencies. For example, a frequency that is dissipated in the load at one rate before heating may dissipate at a different rate (higher or lower) after some heating or movement of the load took place.